Hi Gaurav,
from what I see on gainbitcoin.com it looks like they are offering cloud mining services. We do not recommend cloud mining at 99Bitcoins, as most of the cases they turn out to be scams or at least not profitable at the end. You can read more about our opinion here. If you are ever in doubt if a site is legit or not I suggest you use the Bitcoin Scam Test tool and try to draw your own conclusion.